In pipelines for transport of natural gas and crude oil over a long distance, a reduction in transportation cost has been a universal need, and efforts have focused on improvement of transport efficiency by increasing the maximum working pressure. The standard approach to increasing maximum working pressure involves increasing the wall thickness of low-strength grade steel linepipe. Due to an increase in structural weight however, this method leads to a reduction in the efficiency of on-site welding as well as a reduction in overall pipeline construction efficiency. An alternative approach is to limit the increase in wall thickness by enhancement of the strength of the linepipe material. For example, the American Petroleum Institute (API) recently standardized X80 grade steel, and X80 grade steel has been put in practical use. "X80" means a yield strength (YS) of at least 551 MPa (80 ksi).
In view of anticipated increases in demand for even higher strength steel, several methods for the manufacture of X100 or higher grade steel have been proposed based on the technique used to manufacture X80 grade steel. For example, such a steel and a method of manufacturing the same have been proposed where the strength and toughness are enhanced through Cu precipitation hardening and refinement of the microstructure (Japanese Patent Application Laid-Open (kokai) No. 8-104922). Other such steels and methods of manufacturing the same have been roposed wherein the strength and toughness are enhanced by increasing Mn content and refinement of the microstructure {European Patent Applications: EP 0753596A1 (WO 96/23083) and EP 0757113A1 (WO 96/23909)}.
However, the above-described steels and methods involve the following problems. The former method, which utilizes Cu precipitation hardening, imparts both high strength and excellent field weldability to steel, but due to the presence of Cu precipitates (.epsilon.-Cu phase) dispersed within the steel matrix, is generally ineffective at imparting sufficient toughness to the steel. Also, when the latter high-tensile-strength steel, which contains Mn in excess of 1 wt. %, is manufactured by the continuous casting process (the CC process), impairment in toughness at the center of thickness of a steel plate tends to occur due to centerline segregation. Steel that cannot be manufactured through the continuous casting process, i.e., steel whose slab must be manufactured through ingot making and blooming, tends to have significantly lower yield than that manufactured through the continuous casting process. Steel prepared through the ingot making process is not desirable for mass-production for use in making line pipes due to the expense associated with the ingot making process.
Furthermore, as is disclosed in U.S. Pat. Nos. 5,545,269, 5,545,270 and 5,531,842, of Koo and Luton, it has been found to be practical to produce superior strength steels having yield strengths of at least about 830 MPa (120 ksi) and tensile strengths of at least about 900 MPa (130 ksi), as precursors to linepipe. The strengths of the steels described by Koo and Luton in U.S. Pat. No. 5,545,269 are achieved by a balance between steel chemistry and processing techniques whereby a substantially uniform microstructure is produced that comprises primarily fine-grained, tempered martensite and bainite which are secondarily hardened by precipitates of .epsilon.-copper and certain carbides or nitrides or carbonitrides of vanadium, niobium and molybdenum.
In U.S. Pat. No. 5,545,269, Koo and Luton describe a method of making high strength steel wherein the steel is quenched from the finish hot rolling temperature to a temperature no higher than 400.degree. C. (752.degree. F.) at a rate of at least 20.degree. C./second (36.degree. F./second), preferably about 30.degree. C./second (54.degree. F./second), to produce primarily martensite and bainite microstructures. Furthermore, for the attainment of the desired microstructure and properties, the invention by Koo and Luton requires that the steel plate be subjected to a secondary hardening procedure by an additional processing step involving the tempering of the water cooled plate at a temperature no higher than the Ac.sub.1 transformation point, i.e., the temperature at which austenite begins to form during heating, for a period of time sufficient to cause the precipitation of .epsilon.-copper and certain carbides or nitrides or carbonitrides of vanadium, niobium and molybdenum. The additional processing step of post-quench tempering in these steels leads to a yield to tensile strength ratio of over 0.93. From the point of view of preferred pipeline design, it is desirable to keep the yield to tensile strength ratio lower than about 0.93, while maintaining high tensile strengths.
One method for solving these problems is to utilize a high nickel content in the steel. U.S. Pat. No. 5,545,269 includes up to 2 wt. % nickel. However, depending on the carbon content and other alloying elements in the steel, using a high nickel content, e.g., greater than about 1.5 wt. %, can impair weldability in girth welding during pipeline construction; additionally, added nickel increases the alloying cost. Thus, an object of the present invention is to provide high-tensile-strength steel, with a good yield to tensile strength ratio, i.e., less than about 0.93, which can be manufactured by the continuous casting process, and which has excellent through-thickness toughness, excellent properties at welded joints, a TS of at least about 900 MPa (130 ksi), an impact energy at -40.degree. C. (-40.degree. F.) (e.g., a vE at -40.degree. C.) of greater than about 120 J (90 ft-lbs). Further objects of this invention are to provide such steels having good weldability, such as no cracking, and having an impact energy at -20.degree. C. (-4.degree. F.) (e.g., a vE at -20.degree. C.) in the heat affected zone (HAZ), or welded joint, of greater than about 70 J (52 ft-lbs).